1. Field of the Invention
The present invention relates to ultrasonic diagnostic apparatuses and ultrasonic diagnostic method that obtain biological information of an object by irradiating ultrasonic pulses into the object, receiving ultrasonic echoes generated in the object, and performing various processing operations. In particular, the present invention relates to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method capable of performing imaging according to a contrast echo method using a contrast medium.
2. Description of the Related Art
Ultrasonic diagnostic apparatuses are apparatuses that obtain biological information, such as tomographic images of tissues of a living body in an object and blood flow images, by irradiating ultrasonic pulses into the object from piezoelectric transducers (ultrasonic transducers) included in an ultrasonic probe, receiving ultrasonic echoes generated in the object with the piezoelectric transducers, and performing various processing operations on the received ultrasonic echoes.
An imaging method employed by such ultrasonic diagnostic apparatuses is an imaging method referred to as a contrast echo method. In the contrast echo method, microbubbles are injected into the blood vessel of the object as a contrast medium to enhance the scattering ultrasonic echoes. More specifically, in imaging according to the contrast echo method, ultrasonic pulses having predetermined frequency spectra are irradiated, and nonlinear components of the ultrasonic echoes reflected from the microbubbles as the contrast medium, are used for visualization.
However, in an imaging technique according to a known contrast echo method, only some of the bubbles injected into the object contribute to the visualization. This is because, when the frequency of the ultrasonic pulses to be irradiated is kept constant, the signal strength of the nonlinear components included in the ultrasonic echoes strongly depends on the radiuses of the bubbles. That is, since the resonant frequencies of the bubbles differ depending on the radiuses thereof, only some of the bubbles having radiuses that are resonant with the frequencies of the transmitted ultrasonic pulses are utilized for the visualization.
Thus, it is desirable to obtain ultrasonic echoes reflected from more bubbles having different radiuses so as to visualize images from the aspect of sensitivity.
On the other hand, the transmitted ultrasonic pulses undesirably destroy some of the bubbles. Accordingly, in the imaging according to the contrast echo method, the significantly low intensity ultrasonic pulses, having amplitude only one-tenth of the ultrasonic pulses normally used for non-contrast imaging, are used for the imaging. Thus, there may be a problem that the sensitivity is insufficient for, particularly, regions deep within the object. In particular, in low MI (mechanical index) mode imaging that uses ultrasonic pulses having low sound pressure in order not to destroy the bubbles, the sensitivity is actually insufficient.
In addition, a technique that uses second harmonic components for the visualization as nonlinear components included in ultrasonic echoes reflected from bubbles has been developed. However, in this technique, tissue harmonic components reflected from tissues of a living body are also visualized, which undesirably makes it difficult to see blood flow contrast images enhanced by the bubbles.